std::unordered_map::operator[]

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Containers library

array (C++11)

vector

deque

forward_list (C++11)

list

set

multiset

map

multimap

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

stack

queue

priority_queue

 

std::unordered_map

Member functions

unordered_map::unordered_map

unordered_map::~unordered_map

unordered_map::operator=

unordered_map::get_allocator

Iterators

unordered_map::beginunordered_map::cbegin

unordered_map::endunordered_map::cend

Capacity

unordered_map::erase

unordered_map::size

unordered_map::max_size

Modifiers

unordered_map::clear

unordered_map::insert

unordered_map::insert_or_assign (C++17)

unordered_map::emplace

unordered_map::emplace_hint

unordered_map::try_emplace (C++17)

unordered_map::erase

unordered_map::swap

Lookup

unordered_map::count

unordered_map::find

unordered_map::equal_range

Bucket interface

unordered_map::begin2

unordered_map::end2

unordered_map::bucket_count

unordered_map::max_bucket_count

unordered_map::bucket_size

unordered_map::bucket

Hash policy

unordered_map::load_factor

unordered_map::max_load_factor

unordered_map::rehash

unordered_map::reserve

Observers

unordered_map::hash_function

unordered_map::key_eq

 

T& operator[]( const Key& key );	(1)	(since C++11)
T& operator[]( Key&& key );	(2)	(since C++11)

Returns a reference to the value that is mapped to a key equivalent to key, performing an insertion if such key does not already exist.

If an insertion is performed, the mapped value is value-initialized (default-constructed for class types, zero-initialized otherwise) and a reference to it is returned.

1) Inserts value_type(key, T())

- key_type must meet the requirements of CopyConstructible.

- mapped_type must meet the requirements of DefaultConstructible. (since C++11)

2) Inserts value_type(std::move(key), T())

- key_type must meet the requirements of MoveConstructible. (since C++11)

- mapped_type must meet the requirements of DefaultConstructible. (since C++11)

If an insertion occurs and results in a rehashing of the container, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is equal to or greater than max_load_factor()*bucket_count().

Parameters

key

-

the key of the element to find

Return value

Reference to the mapped value of the new element if no element with key key existed. Otherwise a reference to the mapped value of the existing element whose key is equivalent to key.

Exceptions

If an exception is thrown by any operation, the insertion has no effect

Complexity

Average case: constant, worst case: linear in size.

Example

This example demonstrates how to modify existing values and insert new values using operator[]:

Run this code

#include <iostream>
#include <unordered_map>
 
int main()
{
    std::unordered_map<char, int> letter_counts {{'a', 27}, {'b', 3}, {'c', 1}};
 
    std::cout << "initially:\n";
    for (const auto &pair : letter_counts) {
        std::cout << pair.first << ": " << pair.second << '\n';
    }
 
    letter_counts['b'] = 42;  // update an existing value
 
    letter_counts['x'] = 9;  // insert a new value
 
    std::cout << "after modifications:\n";
    for (const auto &pair : letter_counts) {
        std::cout << pair.first << ": " << pair.second << '\n';
    }
}

Output:

initially:
a: 27
b: 3
c: 1
after modifications:
a: 27
b: 42
c: 1
x: 9

The following example counts the occurrences of each word in a vector of strings:

Run this code

#include <string>
#include <iostream>
#include <vector>
#include <unordered_map>
 
int main()
{
    std::vector<std::string> words = { 
        "this", "sentence", "is", "not", "a", "sentence",
	"this", "sentence", "is", "a", "hoax" 
    };
 
    std::unordered_map<std::string, size_t>  word_map;
    for (const auto &w : words) {
        ++word_map[w];
    }
 
    for (const auto &pair : word_map) {
        std::cout << pair.second
	          << " occurrences of word '"
	          << pair.first << "'\n";
    }
}

Output:

1 occurrences of word 'hoax'
2 occurrences of word 'this'
2 occurrences of word 'a'
2 occurrences of word 'is'
1 occurrences of word 'not'
3 occurrences of word 'sentence'

See also

at	access specified element with bounds checking (public member function)
insert_or_assign (C++17)	inserts an element or assigns to the current element if the key already exists (public member function)